Recently, services to handle large-capacity contents such as a video have been rapidly expanded because of the growth of the Internet and the like. This leads to a growing demand for a large-capacity backbone network; consequently, it becomes important to use a finite optical spectral region more efficiently. One of technologies to use an optical spectral region efficiently is a wavelength division multiplexing (WDM) transmission system. In the WDM system, a plurality of signal light beams with their center wavelengths different from each other are combined, amplified to a desired level in an optical amplifier, and then output to an optical fiber transmission line.
In general, an optical amplifier has a wavelength-dependent gain, and particularly, the wavelength dependence prominently arises if an input level of an optical signal is reduced. In this case, an output level of an optical signal becomes dependent on wavelength. Patent Literature 1 discloses a technology to maintain a constant gain of an optical amplifier by disposing a correction light source and, if the number of optical signals composing a WDM signal decreases, by introducing correction light into the WDM signal instead. FIG. 11A illustrates an example of an output level without correction light introduced, and FIG. 11B illustrates an example of an output level with correction light introduced, if the number of optical signals decreases.
In FIG. 11A and FIG. 11B, if optical signals of wavelengths λ3 to λ7 are lost, the wavelength dependence of the gain of an optical amplifier prominently arises if the correction light is not introduced, and output levels of main signal light with λ1, λ2, and λ8 becomes dependent on wavelength. In contrast, the gain of the optical amplifier can be flattened by introducing correction light with λa to λe corresponding to the lost optical signals with the wavelengths λ3 to λ7; consequently, the output levels of main signal light with λ1, λ2, and λ8 become unaffected by the gain.